Re: [PATCH] bpf: defer printk() inside __bpf_prog_run()

[Tetsuo Handa]

On 2024/06/26 0:47, John Ogness wrote:
> On 2024-06-26, Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
>> On 2024/06/25 23:17, John Ogness wrote:
>>> On 2024-06-25, Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
>>>> syzbot is reporting circular locking dependency inside __bpf_prog_run(),
>>>> for fault injection calls printk() despite rq lock is already held.
>>>>
>>>> Guard __bpf_prog_run() using printk_deferred_{enter,exit}() (and
>>>> preempt_{disable,enable}() if CONFIG_PREEMPT_RT=n) in order to defer any
>>>> printk() messages.
>>>
>>> Why is the reason for disabling preemption?
>>
>> Because since kernel/printk/printk_safe.c uses a percpu counter for deferring
>> printk(), printk_safe_enter() and printk_safe_exit() have to be called from
>> the same CPU. preempt_disable() before printk_safe_enter() and preempt_enable()
>> after printk_safe_exit() guarantees that printk_safe_enter() and
>> printk_safe_exit() are called from the same CPU.
> 
> Yes, but we already have cant_migrate(). Are you suggesting there are
> configurations where cant_migrate() is true but the context can be
> migrated anyway?

No, I'm not aware of such configuration.

Does migrate_disable() imply preempt_disable() ?
If yes, we don't need to also call preempt_disable().
My understanding is that migration is about "on which CPU a process runs"
and preemption is about "whether a different process runs on this CPU".
That is, disabling migration and disabling preemption are independent.

Is migrate_disable() alone sufficient for managing a percpu counter?
If yes, we don't need to also call preempt_disable() in order to manage
a percpu counter.